The further development of manually shifted transmissions and differentials is still an area of primary concern in the automobile industry. Along with increasing the ease of shifting gears; reducing the dimensions of the transmission; and lowering the power losses in transmissions, measures for containing or reducing the noises from the transmission continue to merit a high priority. To date, measures for reducing transmission noise have extended to achieving ever higher precision in toothing geometries and focusing upon the noise-damped mounting of the transmission unit as a whole focusing to adjoining units in the motor vehicle. Development work is also continuing on constructional measures to facilitate synchronization of transmission components upon shifting so as to reduce the shifting noises which actually occur.
In the noise-damping construction of machines and other units, extensive use is made of components with a composite structure. That is to say, the components made up of composites display zones of different material composition and/or structure. Composite structures include both components which exhibit improvement of the surface of the material, e.g. a surface coating, and those in which a number of individual parts composed of different materials are subjected to a joining operation, e.g. by welding, adhesive bonding or a mechanical shrink-on operation, in order to construct a single component.
It is, in particular, also known, by using sinter metallurgy, to integrally convert components produced by various methods to components with zones of different materials and/or structures, or to otherwise reconfigure them by appropriate after treatments.
The aforementioned measures are a means of reconciling the different property requirements in the various zones of a component, be these of a mechanical or chemical nature, against the commercial requirement of minimizing material and process costs. In many cases, it is possible in this way to stretch the limits of what is technically feasible.
Examples of this can be found from the nearest-related prior art which is to be taken into account.
DE 23 10 536 A1 (entitled "Process for the production of objects made of composite metal") describes a process in which a core comprising already compacted metal powder is surrounded with a metal powder--in general of a different type--and the unit thus formed is compacted. The aim of the process is to give a product with a hard, abrasion-resistant exterior and a relatively tough, easily machinable interior (page 2, paragraph 3). Hubs with a hard central region or gear cutters with a "foot of relatively tough material" and an "extremely hard toothed surface" are mentioned as examples of such products.
Another example is DE 30 07 008 C2 entitled "wear-resistant part for internal combustion engines and a process for its manufacture". The wear-resistant part comprises a main body of iron or steel and a sintered body composed of a specific iron alloy and having certain void characteristics, joined intimately to the main body by sintering. Here too, the aim of the measures is to provide a component "with a high toughness in its body and a high abrasion-resistance in at least one portion of its surface" (column 2, lines 41-44).
Common to the above-described previously known embodiments of employing a sintered shaped part as a component is the fact that although they display high-strength rim zones having few voids or being largely void-free, they do not have a highly porous central region composed of comparatively ductile material. On the contrary, these processes presented are only suitable of producing, in an economical manner, sintered bodies with 90% or more of the theoretical density, even in the central region. They are not aimed at guaranteeing high porosity in the central region in combination with a high rim density.
U.S. Pat. No. 2,561,579 describes a gearwheel produced by powder metallurgy and composed of an impregnated ferrous material. Partial areas of the gearwheel are impregnated with a melt consisting of copper or copper alloys in order to increase the strength in these areas or to modify the physical properties of these partial areas. The problem of noise damping is neither addressed by the patent nor achieved in a closely related manner by means of the embodiments specified in the patent.